1. The Field of the Invention
The present invention relates to an electrooptic device, and more particularly relates to an electrooptic device that changes the refractive index of an electrooptic crystal by controlling the electric field of the crystal, so that the forwarding direction of light can be changed, or the phase of light can be changed.
2. The Relevant Technology
At present, requests for an optical control device that deflects a laser beam have increased for video apparatuses, such as projectors, laser printers, confocal microscopes having a high resolution, barcode readers, etc. As optical deflection techniques, a technique for rotating a polygon mirror, a technique for employing a galvano mirror to control the deflected direction of light, a diffraction technique that employs the acousto-optic effect, and a micro machine technique called the MEMS (Micro Electro Mechanical System) have been proposed.
As for a polygon mirror, a mirror having the shape of a polyhedron is mechanically rotated, and the reflection direction of a laser beam is sequentially changed to deflect light. Since a method employing a polygon mirror utilizes mechanical rotations, the rotational speed is limited. That is, the acquisition of revolutions equal to or greater than 10000 rpm is difficult for a polygon mirror, and there is a fault in that a polygon mirror is not appropriate for an application required for a rapid operation. A method employing a polygon mirror has been utilized for the deflection of the laser beam of a laser printer. However, the limit imposed by the rotational speed of a polygon mirror is a bottleneck when it comes to increasing the printing speed of a printer. In order to further increase the printing speed of a printer, a faster optical deflection technique is required.
A galvano mirror is employed for a laser scanner, etc., that deflects and scans a laser beam. A conventional practical galvano mirror has, for example, a magnetic path formed by a moving iron core, which is used instead of a moving coil arranged in a magnetic field, and a magnetic member, around which two permanent magnets and four magnetic poles are arranged.
When the magnetic fluxes between the magnetic poles are changed by the magnitude and the direction of a current that flows across a drive coil that is wound around the magnetic member, a reflecting mirror is moved via the moving iron core and the laser beam is deflected and scanned. The method employing a galvano mirror can perform a rapid operation. However, since the drive coil of a conventional galvano mirror is provided by a machine winding, downsizing is difficult. Therefore, it is difficult for the sizes of a laser scanning system employing a galvano mirror and a laser application apparatus that employs this system to be further reduced. Furthermore, there is a fault that power consumption is large. There is another fault in that a rapid operation can not be performed within a cycle of the MHz unit.
An optical deflector of an optical diffraction type that employs the acousto-optic effect has been put to practical use. However, a method employing this optical deflector of an optical diffraction type consumes a large amount of power and downsizing is difficult. Further, there is a fault in that it is difficult to obtain a large deflection angle and to perform a rapid operation. In addition, since a method employing the MEMS electrostatically drives a fine mirror as an optical deflection device, several tens of .mu.m is the limit placed on the response.
Conventionally, various optical function parts employing an electrooptic crystal have been put to practical use. These optical function parts employ a phenomenon such that, upon the application of a voltage to an electrooptic crystal, the refractive index of the crystal is changed by the electrooptic effect. Thus, as means for solving the above described problems, a technique has been developed whereby a voltage is applied to the electrodes of an electrooptic crystal, and a beam is deflected by the electrooptic effect (see, for example, patent document 1). Furthermore, a technique has been developed whereby a beam is deflected using an electrooptic crystal that is processed in a prism shape, or an electrooptic crystal wherein electrodes having a prism shape are formed (see, for example, patent document 2). When a voltage is applied to the electrodes of the electrooptic crystal, the refractive index can be changed because of the electrooptic effect. By using the method that employs electrodes shaped like a prism, an area where the refractive index is changed and an area where a voltage is not applied, and a refractive index is not changed, are produced in the electrooptic crystal. Due to a refractive index difference at the boundary of the two areas, a beam is deflected, and a deflection angle is obtained.
By using the method employing the electrooptic crystal, a response up to the speed limit of the electrooptic effect is available, and a response exceeding one GHz can be obtained.
In the past, reports of using LiNbO3 (hereinafter referred to as an LN crystal) and PLZT were submitted as optical deflection devices employing an electrooptic crystal. However, since a device employing the LN crystal produces only a small electrooptic effect, there is a fault in that only a deflection angle of about 3 mrad is obtained by applying a voltage of about 5 kV/mm. Further, also for a device using PLZT, a deflection angle of about 45 mrad is the limit, relative to the application of an electric field of 20 kV/mm (see, for example, non-patent document 1).
However, according to the conventional method, there is only a small change of the refractive index in each prism area due to the electrooptic effect, and the deflection angle due to the refractive index change is also small. Therefore, in order to obtain a large deflection angle, a plurality of prisms must be arranged for the conventional method. However, in a case wherein a plurality of prisms are arranged, there is a problem in that, when light enters the prisms at a large incident angle, a desired resolution cannot be obtained.
On the other hand, an optical phase modulator employing an electrooptic crystal changes the refractive index of the crystal to change the speed at which light passes through the crystal, and to change the phase of the light. Further, when the electrooptic crystal is located on one of the optical waveguide paths of a Mach-Zehnder interferometer and a Michelson interferometer, the light intensity of the output of the interferometer is changed in accordance with a voltage applied to the crystal. These interferometers can be employed as optical switches or optical modulators.
FIG. 1 shows the structure of a conventional optical phase modulator employing an electrooptic crystal. In the optical phase modulator, a positive electrode 2 and a negative electrode 3 are formed on opposite faces of the block of an electrooptic crystal 1. The crystal axes x, y and z of the electrooptic crystal 1 are defined as shown in FIG. 1. The change in the refractive index due to the electrooptic effect is provided by the linear Pockels effect and the quadratic Kerr effect.
In the case of the quadratic Kerr effect, s11 is an electrooptic constant for vertically polarized light, i.e., for the polarization direction relative to the x axial direction in FIG. 1. The change in a phase when a voltage V is applied between the positive electrode 2 and the negative electrode 3 is provided by the following expression.
                              [                      Expression            ⁢                                                  ⁢            1                    ]                ⁢                                                                                                ϕ          x                =                                            π              ⁢                                                          ⁢                              n                3                            ⁢                              Ls                11                                      λ                    ⁢                                    (                              V                d                            )                        2                                              (        1        )            
Here, n denotes the refractive index of the electrooptic crystal 1, L denotes a light propagation direction, i.e., the length of the electrooptic crystal 1 in the z axial direction in FIG. 1, λ denotes the wavelength of light, and d denotes the interval between the positive electrode 2 and the negative electrode 3. s12 is an electrooptic constant for horizontally polarized light, i.e., for a polarization direction relative to the y axial direction in FIG. 1, and the change in a phase when a voltage V is applied between the positive electrode 2 and the negative electrode 3 is obtained by using the following expression.
                              [                      Expression            ⁢                                                  ⁢            2                    ]                ⁢                                                                                                ϕ          y                =                                            π              ⁢                                                          ⁢                              n                3                            ⁢                              Ls                12                                      λ                    ⁢                                    (                              V                d                            )                        2                                              (        2        )            
A half-wave voltage is employed as an index that represents the efficiency of the optical phase modulator. A half-wave voltage is a voltage that is required to change the phase of light by it radian, and is provided by the following expression.
                              [                      Expression            ⁢                                                  ⁢            3                    ]                ⁢                                                                                                V          π                =                                            λ              ⁢                                                          ⁢                              d                2                                                                    n                3                            ⁢                              Ls                ij                                                                        (        3        )            
Next, an explanation will be given for a light intensity modulator that is constituted by combining an optical phase modulator, a polarizer and an analyzer. FIGS. 2A and 2B show the structure of a conventional light intensity modulator. As shown in FIG. 2A, a positive electrode 2 and a negative electrode 3 are formed on opposite faces of an electrooptic crystal 1. A polarizer 4 is located on the incidence side of the electrooptic crystal 1, and an analyzer 5 is located on the emittance side. Of the field elements of light that is passed through the polarizer 4, the element parallel to the x axis is defined as Ex, and the element parallel to the y axis is defined as Ey. In a case wherein the polarization angle of the polarizer 4 is 45 degrees relative to the x axis of the electrooptic crystal 1, Ex=Ey.
The changes in the phases of Ex and Ey upon the application of a voltage V between the positive electrode 2 and the negative electrode 3 are respectively obtained by expressions (1) and (2). In a case wherein the polarization angle of the analyzer 5 is 45 degrees relative to the x axis of the electrooptic crystal 1, the intensity of the output light that is passed through the analyzer 5 is provided by the following expression.
                              [                      Expression            ⁢                                                  ⁢            4                    ]                ⁢                                                                                                                          I              =                            ⁢                                                                                                                                                              E                          x                                                                          2                                                                    ⁢                                              ⅇ                                                  j                          ⁢                                                                                                          ⁢                                                      ϕ                            x                                                                                                                +                                                                                            E                          y                                                                          2                                                                    ⁢                                              ⅇ                                                  j                          ⁢                                                                                                          ⁢                                                      ϕ                            yx                                                                                                                                                                2                                                                                        =                            ⁢                                                                    E                    x                    2                                    2                                +                                                      E                    y                    2                                    2                                +                                                      E                    x                                    ⁢                                      E                    y                                    ⁢                                      cos                    ⁡                                          (                                                                        ϕ                          x                                                -                                                  ϕ                          y                                                                    )                                                                                                                                              =                            ⁢                                                                    E                    x                    2                                    2                                +                                                      E                    y                    2                                    2                                +                                                      E                    x                                    ⁢                                      E                    y                                    ⁢                  cos                  ⁢                                      {                                                                                            π                          ⁢                                                                                                          ⁢                                                      n                            3                                                    ⁢                          L                                                λ                                            ⁢                                              (                                                                              s                            11                                                    -                                                      s                            12                                                                          )                                            ⁢                                                                        (                                                      V                            d                                                    )                                                2                                                              }                                                                                                          (        4        )            
In a case wherein Ex and Ey are equal,
                              E          x                =                              E            y                    =                      E                          2                                                          [                  Expresion          ⁢                                          ⁢          5                ]            is employed, and the light intensity is provided by the following expression.
                              [                      Expression            ⁢                                                  ⁢            6                    ]                ⁢                                                                                                                          I              =                            ⁢                                                                    E                    2                                    2                                ⁡                                  [                                      1                    +                                          2                      ⁢                      cos                      ⁢                                              {                                                                                                            π                              ⁢                                                                                                                          ⁢                                                              n                                3                                                            ⁢                              L                                                        λ                                                    ⁢                                                      (                                                                                          s                                11                                                            -                                                              s                                12                                                                                      )                                                    ⁢                                                                                    (                                                              V                                d                                                            )                                                        2                                                                          }                                                                              ]                                                                                                                        =                                ⁢                                                      E                    2                                    ⁢                                      sin                    2                                    ⁢                                      {                                                                                            π                          ⁢                                                                                                          ⁢                                                      n                            3                                                    ⁢                          L                                                λ                                            ⁢                                              (                                                                              s                            11                                                    -                                                      s                            12                                                                          )                                            ⁢                                                                        (                                                      V                            d                                                    )                                                2                                                              }                                                              ⁢                                                                                                      (        5        )            
In this manner, as shown in FIG. 2B, the intensity of the output light that is passed through the analyzer 5 can be modulated between 0% to 100%, in accordance with the voltage V. As an index that indicates the efficiency of the light intensity modulator, a semi-half voltage that changes the intensity of the output light from 0% to 100% is represented by the following expression.
                              [                      Expression            ⁢                                                  ⁢            7                    ]                ⁢                                                                                                V          x                =                                            λ              ⁢                                                          ⁢                              d                                  2                  ⁢                                                                                                                                            n                3                            ⁢                              L                ⁡                                  (                                                            s                      11                                        -                                          s                      12                                                        )                                                                                        (        6        )            
However, since the conventional electrooptic crystal has only a small electrooptic constant, in order to constitute an optical phase modulator and a light intensity modulator for practical use, a half-wave voltage of a kV order must be employed. Since a great load is imposed on a drive circuit for fast modulation of the voltage of a kV order, there is a problem in that increasing the size of an apparatus can not be avoided. Further, there is also a problem in that, when a voltage of a kV order is modulated at a high speed, high frequency noise occurs, and will enter a peripheral device.
One objective of the present invention is to provide an electrooptic device having a simple arrangement that can efficiently increase the deflection of abeam. Further, another objective of the present invention is to provide an electrooptic device having a simple arrangement that can efficiently modulate the phase of light.    Patent Document 1: Japanese Patent Laid-Open No. Hei 10-239717    Patent Document 2: Japanese Patent Laid-Open No. Hei 09-159950    Non-Patent Document 1: Akio Sugama, et al., “Development of EO waveguide Path Deflection Optical Switch”, Technical Report of The Institute of Electronics, Information and Communication Engineers, PN2004-59, p. 61 to 64, published October, 2004 by the Institute of Electronics, Information and Communication Engineers Association.    Non-Patent Document 2: Toshihiro Itoh, Masahiro Sasaura, Seiji Toyoda, Katsue Manabe, Koichiro Nakamura and Kazuo Fujiura, “High-frequency response of electro-optic single crystal KTaxNbl-xO3 in paraelectric phase,” in Conference on Lasers and Electro-Optics/Quantum Electronics and Laser Science and Photonic Applications, Systems and Technologies 2005 (Optical Society of America, Washington, D.C., 2005), JTuC 36    Non-Patent Document 3: P. S. Chen, et. al., “Light Modulation and Beam Deflection with Potassium Tantalate-Niobate Crystals,” Journal of Applied Physics, 1966, Vol. 37, no. 1, pp. 388-398